The Toclafane
The Toclafane first appeared at the end of Series 3 in The Sound of Drums (Series 3 : Episode 12) and Last of the Time Lords (Series 3 : Episode 13). They were called Toclafane by the Master, who named them after a Gallifreyan equivalent of Earth’s “Boogeyman”. The Toclafane were futuristic humans that had evolved into spherical cyborgs to survive. The humans that evolved were told to “Come to Utopia” but found only a dark, cold and inhospitable world that wasn’t fit for humanity to live on. The Toclafane were then found by the Master, who befriended them and brought them back to present day Earth to use as his army.
The Master built a paradox machine that allowed the Toclafane to kill their ancestors without erasing themselves from time. The Toclafane weapons consisted of lasers that could turn humans to ash and sharp metal spikes that could slice up humans. The technology of the spheres allowed the Toclafane to travel through space and hover in the atmosphere unharmed. Inside the spheres however, rested the human head of whichever human had evolved into that particular Toclafane. The invasion of Earth by the Toclafane, on the Master’s orders, was successful, subjugating and killing humanity, forcing them to build a fleet of 200,000 rockets to take over the rest of the Universe. The Master intended to build a new Time Lord empire and use the Toclafane as his foot soldiers.
The Toclafane were defeated when Captain Jack Harkness destroyed the paradox machine built to keep the Toclafane out of their own time zone. The destruction of the paradox machine resulted in the Toclafane returning to the end of the Universe, with the human race having no recollection that they had invaded at all, with the exception of those aboard the aircraft carrier Valiant. There, the Toclafane are presumed to have died out along with their cousins on Malcassairo, bringing humanity to extinction.